Document non-use of certain operand types
Document non-use of certain operand types

Operand class enum values not used in the syntax table:
- Image channel order: only used to describe a return value
- Image channel type: only used to describe a return value
- "Image operands": used to make a section in the spec
  to describe values used to construct an optional image id,
  but does not itself appear in the syntax table.

Removes some TODOs from opcode.cpp.

dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools,umar456/SPIRV-Tools,antiagainst/SPIRV-Tools,antiagainst/SPIRV-Tools,antiagainst/SPIRV-Tools,LukasBanana/SPIRV-Tools,KhronosGroup/SPIRV-Tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,umar456/SPIRV-Tools,LukasBanana/SPIRV-Tools,KhronosGroup/SPIRV-Tools,LukasBanana/SPIRV-Tools,dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,dneto0/SPIRV-Tools,pierremoreau/SPIRV-Tools,greg-lunarg/SPIRV-Tools,dneto0/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,umar456/SPIRV-Tools,pierremoreau/SPIRV-Tools,antiagainst/SPIRV-Tools,antiagainst/SPIRV-Tools,pierremoreau/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,greg-lunarg/SPIRV-Tools,dneto0/SPIRV-Tools,pierremoreau/SPIRV-Tools,pierremoreau/SPIRV-Tools,dneto0/SPIRV-Tools,dneto0/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,greg-lunarg/SPIRV-Tools,KhronosGroup/SPIRV-Tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,dneto0/SPIRV-Tools,pierremoreau/SPIRV-Tools